The giving tree: How planting trees saves the green

Monday

Apr 15, 2013 at 12:01 AMApr 15, 2013 at 12:11 PM

Planting trees can save money on cooling costs - if they are planted in the right places.

Michael De Groote

SACRAMENTO, Calif. - When California shut down nuclear power plants in 1989, it lost half its electrical capacity overnight, says Misha Sarkovich, program manager at the Sacramento Municipal Utility District.
"We began to aggressively invest in conservation programs," Sarkovich says. "One of the first programs where we thought outside of the box was the shade tree program."
It turns out that trees are the answer.
Since 1990, SMUD has planted more than half a million trees to reduce energy costs - including trees at about 175,000 single-family homes. Strategically locating trees can save money, and with spring and tree-planting season beginning, people can make a modest investment that can improve the bottom line.
Benefits
Greg McPherson, research forester at the Pacific Southwest Research Station in Albany, Calif., says trees have many benefits, but are most helpful for heating and cooling homes.
An analysis by McPherson in The Electricity Journal in 1993 showed that using three 25-foot-tall trees (two on the west and one on the east) on a two-story structure gave a yearly heating and cooling energy savings ranging from 2 percent to 9 percent. The warmer the climate, the better the savings.
A 2008 study at Auburn University found that electricity consumption and costs are 11.4 percent less during the summer when a house has just 17.5 percent heavy shade coverage, compared to a house with no shade.
The Auburn study also found (using rates at the time in Auburn, Ala.) the 11.4 percent energy savings would equal $31 to $33 per month. Even light shade, if it covered about half of a house, saved 10.3 percent on cooling costs.
The savings do not just come from shading, but also from "evapotranspiration," the transfer of water vapor from plants to the air around them - essentially acting like evaporative coolers. Trees also break up the flow of wind, slowing it down.
Other benefits are reduced water run-off. In 2008, the Indiana Department of Natural Resources commissioned a study that looked at the various benefits of trees. The study found that 31 percent of the value of planting trees for the community at large was slowing the runoff of storm water. Slowing runoff reduces water treatment costs and reduces soil erosion.
Twelve percent of the benefits was reducing energy consumption. More than half of the benefits were aesthetic and other benefits, the study said.
To a lesser extent, trees also improved air quality (4 percent of the total benefit) and CO2 absorption (1 percent). They clean the air by absorbing gaseous pollution into their leaves and by trapping particulate pollution.
Sarkovich says trees are also good for taking CO2 out of the air, which would probably double the cost effectiveness of planting trees, he says. The ecosmartlandscapes.org website helps people calculate the ecological impact of their trees and plants.
People can search at energysavingtrees.arborday.org for more information about saving money with trees and to see if their local utility provides free shade trees. Utilities in seven states plus Washington, D.C., are partners with the Arbor Day Foundation. Other communities may also have information and programs.
The SMUD's shade tree program (which provides free trees to customers in the Sacramento area) has saved an average of $12 per year for each mature tree. "We think a combination of strategically planted trees cans save up to $40 of cooling costs, if done right," Sarkovich says.
West is best
The primary way to plant the trees right deals with location, McPherson says. "If you can locate one tree anywhere, the west side of the home is best," he says. "The east side would be your second preference."
The reason why the west gives the most savings is because it is warmest in the afternoon as the sun is getting lower in the western sky. The sun is shining through western windows. That is also the time that people are coming home from work and turning on their air conditioning.
"You get the biggest bang for your buck when you plant in the west," McPherson says.
It is important also to plant a deciduous tree - a tree that sheds its leaves in the fall. That is because having the sun heat up the home is bad in the summer, but important in the winter. Planting an evergreen in that location will increase the heating load in winter.
Planting trees on the south doesn't have the same beneficial effect economically. McPherson recommends choosing trees on the south that drop leaves early and leaf out late in the spring so the sun can start warming the house as the weather turns cooler.
It is true that the branches of trees that shed their summer leaves will give some shade to a home in the winter, but Sarkovich says the savings benefits accumulated in the summer by cooling the home far exceed any loss of the sun's warmth in the winter.
Smaller trees that shade air conditioners may also help. Sarkovich says shielding air conditioners] from the direct rays of the sun may extend the life of the condenser and save some energy because the air conditioner doesn't have to work as hard.
Of course it may take a few years for trees to mature enough to have a bigger impact. But there are other benefits.
"Just seeing a tree and urban greenery reduces stress and helps people to relax and recover from fatigue," says McPherson.
And that is a different type of cool.
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